What do you thing is the best OS for MCServer?

[xoft]

I'd say 64-bit is better, because if your server ever grows, you can give it more memory beyond the 2 GiB available to 32-bit programs. Other than that, no idea if there's any difference at all. As far as I know, compiling MCServer has been tested on Debian and Ubuntu, so CentOS has a slight disadvantage here because, however unlikely, it just might not work.

I don't think anyone has ever benchmarked MCServer performance on different OSes while on the same hardware, and I believe the performance differences would be negligible.